1. Field of the Invention
The present invention relates generally to a production or manufacturing of high cleaness extra low carbon steel.
2. Description of the Background Art
Conventionally, extra low carbon steel having carbon content less than 0.006% has been produced by preparing molten steel having carbon in a range of greater than or equal to 0.01% and less than or equal to 0.06% by means of a refining furnace, which is no subject deoxidation process and supplied in a form of rimmed steel, and by performing vacuum decarbonization process. In this conventional process of production of the extra low carbon steel, iron concentration T.Fe in iron oxide in a slag remained in a laddle has been in a range of 8% to 25%.
In the process of vacuum decarbonization, reaction of carbon and oxygen is caused for forming carbon monoxide vapor. For example, it has been known that, for decarbonizing the molten steel having carbon content of 400 ppm to reduce carbon content at 30 ppm, 493 ppm of oxygen is required. Oxygen is supplied by oxygen contained in the molten steel and oxygen contained in iron oxide in the slag.
For this purpose, it has been required to maintain high iron concentration (T.Fe) in the slag in the laddle. In decarbonization process for non-deoxidized molten steel by way of RH vacuum degassing apparatus, reaction caused between the molten steel and the slag is relatively small to maintain T.Fe in the slag relatively high even after decarbonization process. The slag can react with impurity or impurities, such as aluminium and so forth, to increase oxygen concentration in the molten steel to degrade cleaness of the produced steel. Furthermore, the slag having high T.Fe flows into a tundish for continuous casting to increase blocking of a continuous casting nozzle.
For this, there has been proposed a technology for reducing T.Fe in slag by supplying a deoxidizing agent in the laddle in Japanese Patent First (unexamined) Publication (Tokkai) Showa No. 59-70710. As will be appreciated, when the deoxidizing agent is supplied to the laddle, it will lead lack of oxygen required for vacuum decarbonization process. Therefore, this prior proposed technology is considered not applicable in practical operation for producing the extra low carbon steel.